Common reference signal design based on semi-uniform pilot spacing and orthogonal cover code

ABSTRACT

Systems and techniques are disclosed to reduce pilot overhead by providing common reference signals coded with cover codes that are orthogonal in time and frequency domains. Common reference signals that are coded by cover codes orthogonal in both domains can be de-spread in both the time and frequency domains for improved resolution and larger pull-in windows for both. Also disclosed is semi-uniform pilot spacing in both the frequency and time domains. In a time domain, a first pilot symbol pair is spaced by a first time interval from each other and a second pilot symbol pair is spaced by a second time interval from the first pair, the second interval being greater than the first. In a frequency domain, a first set of pilot symbols is densely placed in a selected frequency band and a second set of pilot symbols is sparsely placed surrounding and including the selected frequency band.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication No. 62/094,721, filed Dec. 19, 2014, and entitled “CommonReference Signal Design Based on Semi-Uniform Pilot Spacing andOrthogonal Cover Code,” the disclosure of which is incorporated byreference herein in its entirety.

TECHNICAL FIELD

This application relates to wireless communication systems, and moreparticularly to semi-uniform pilot spacing and orthogonal spreading forreducing pilot signal overhead while still maintaining sufficientdensity for channel estimation and other purposes.

BACKGROUND

Reference signals, such as pilots, may be inserted in a transmitted datastream to assist a receiving entity with various functions, includingnot only channel estimation but also timing and frequency offsetacquisition. A reference signal typically includes one or moremodulation symbols known to both the transmitting entity and thereceiving entity that are transmitted in a known manner. Since referencesignals represent overhead in a system, it is desirable to minimize theamount of system resources used to transmit reference signals (e.g.,pilots).

Conventional systems employ various types of reference signals, withvarying fixed structures, to provide sufficient measurements andestimations for adaptive multi-antenna operation. For example, a commonreference signal is a signal used by many if not all transmitters in anetwork to facilitate channel estimation. The common reference signalcan employ a fixed pilot structure that provides an adequate number anddistribution of pilot symbols for most receiving entities under mostchannel conditions. However, this approach results in a common overheadfor all receiving entities. The common overhead becomes difficult toscale up to large numbers of transmit ports (e.g., in massivemultiple-input, multiple-output (MIMO)) as well as results in a densepilot structure that can cause pilot pollution in partially loaded orunloaded cells.

Another type of known reference signal is a channel state informationreference signal (CSI-RS) that employs a fixed pilot structure that issignificantly sparser than that used for the common reference signal.The CSI-RS is useful for estimating channel quality in frequencies otherthan those assigned to specific user equipment (UEs) in a given cell.Although CSI-RS results in a smaller overhead, the spacing can be toolarge in the time domain to train a frequency tracking loop. The CSI-RSmay also result in an aliased channel energy response under a long delayspread channel. Thus, there is a need for techniques to providereference signal spacing and structures that provide more information toestimate to channel conditions.

SUMMARY

In one aspect of the disclosure, a method for wireless communicationincludes applying, by a processor of a wireless communications device, afirst cover code to a first pilot sequence to produce a first set ofpilot symbols comprising a first common reference signal and a secondcover code to a second pilot sequence to produce a second set of pilotsymbols comprising a second common reference signal, wherein the firstcover code and the second cover code are orthogonal to each other intime and frequency domains; delivering the first common reference signalusing a first number of resource elements to a first transmit port;delivering the second common reference signal using a second number ofresource elements to a second transmit port; transmitting, from thefirst transmit port, the first common reference signal; andtransmitting, from the second transmit port, the second common referencesignal.

In an additional aspect of the disclosure, a wireless communicationsdevice includes a processor configured to: apply a first cover code to afirst pilot sequence to produce a first set of pilot symbols comprisinga first common reference signal and a second cover code to a secondpilot sequence to produce a second set of pilot symbols comprising asecond common reference signal, wherein the first cover code and thesecond cover code are orthogonal to each other in time and frequencydomains; deliver the first common reference signal using a first numberof resource elements for transmission; deliver the second commonreference signal using a second number of resource elements fortransmission; and a transceiver comprising a first transmit port and asecond transmit port, the first transmit port configured to transmit thefirst common reference signal and the second transmit port configured totransmit the second common reference signal.

In an additional aspect of the disclosure, a computer-readable mediumhaving program code recorded thereon includes program code comprisingcode for causing a wireless communications device to apply a first covercode to a first pilot sequence to produce a first set of pilot symbolscomprising a first common reference signal and a second cover code to asecond pilot sequence to produce a second set of pilot symbolscomprising a second common reference signal, wherein the first covercode and the second cover code are orthogonal to each other in time andfrequency domains; code for causing the wireless communications deviceto deliver the first common reference signal using a first number ofresource elements to a first transmit port; code for causing thewireless communications device to deliver the second common referencesignal using a second number of resource elements to a second transmitport; code for causing the wireless communications device to transmit,from the first transmit port, the first common reference signal; andcode for causing the wireless communications device to transmit, fromthe second transmit port, the second common reference signal.

In an additional aspect of the disclosure, a method for wirelesscommunication includes receiving, at a wireless communications device, afirst set of pilot symbols using a number of resource elements andspread with a first cover code; receiving, at the wirelesscommunications device, a second set of pilot symbols using a secondnumber of resource elements and spread with a second cover code, thefirst and second cover codes being orthogonal to each other in time andfrequency domains, the first and second set of pilot symbols comprisinga common reference signal; and de-spreading the first and second sets ofpilot symbols in the frequency domain to recover at least two pilotobservations in the time domain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wireless communication network in accordance withvarious aspects of the present disclosure.

FIG. 2 is a block diagram illustrating an exemplary transmitter systemin accordance with various aspects of the present disclosure.

FIG. 3A illustrates a downlink frame structure for a common referencesignal multiplexing design using multiple transmit ports in a wirelesscommunication network in accordance with various aspects of the presentdisclosure.

FIG. 3B illustrates a downlink frame structure for a common referencesignal multiplexing design using multiple transmit ports in a wirelesscommunication network in accordance with various aspects of the presentdisclosure.

FIG. 4 is a protocol diagram illustrating some signaling aspects betweena base station and a UE for supporting common reference signalsmultiplexed using multiple transmit ports in accordance with variousaspects of the present disclosure.

FIG. 5A is a flowchart illustrating an exemplary method for generatingand multiplexing a common reference signal using multiple transmit portsin accordance with various aspects of the present disclosure.

FIG. 5B is a flowchart illustrating an exemplary method for utilizing acommon reference signal received at multiple receiver ports inaccordance with various aspects of the present disclosure.

FIG. 6 illustrates a downlink frame structure for a common referencesignal in a semi-uniform time domain arrangement in accordance withvarious aspects of the present disclosure.

FIG. 7A is a flowchart illustrating an exemplary method for generatingand transmitting common reference signals in a semi-uniform time domainarrangement in accordance with various aspects of the presentdisclosure.

FIG. 7B is a flowchart illustrating an exemplary method for utilizingcommon reference signals received in a semi-uniform time domainarrangement in accordance with various aspects of the presentdisclosure.

FIG. 8 illustrates common reference signal spacing in a semi-uniformfrequency domain arrangement in accordance with various aspects of thepresent disclosure.

FIG. 9A is a flowchart illustrating an exemplary method for generatingand transmitting common reference signals in a semi-uniform frequencydomain arrangement in accordance with various aspects of the presentdisclosure.

FIG. 9B is a flowchart illustrating an exemplary method for utilizingcommon reference signals received in a semi-uniform frequency domainarrangement in accordance with various aspects of the presentdisclosure.

FIG. 10 illustrates common reference signal spacing in both asemi-uniform frequency domain arrangement and a semi-uniform time domainarrangement in accordance with various aspects of the presentdisclosure.

DETAILED DESCRIPTION

The detailed description set forth below, in connection with theappended drawings, is intended as a description of variousconfigurations and is not intended to represent the only configurationsin which the concepts described herein may be practiced. The detaileddescription includes specific details for the purpose of providing athorough understanding of the various concepts. However, it will beapparent to those skilled in the art that these concepts may bepracticed without these specific details. In some instances, well-knownstructures and components are shown in block diagram form in order toavoid obscuring such concepts.

The techniques described herein may be used for various wirelesscommunication networks such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA andother networks. The terms “network” and “system” are often usedinterchangeably. A CDMA network may implement a radio technology such asUniversal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includesWideband CDMA (WCDMA) and other variants of CDMA. cdma2000 coversIS-2000, IS-95 and IS-856 standards. A TDMA network may implement aradio technology such as Global System for Mobile Communications (GSM).An OFDMA network may implement a radio technology such as Evolved UTRA(E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, Flash-OFDMA, etc. UTRA and E-UTRA are part ofUniversal Mobile Telecommunication System (UMTS). 3GPP Long TermEvolution (LTE) and LTE-Advanced (LTE-A) are new releases of UMTS thatuse E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A and GSM are described indocuments from an organization named “3rd Generation PartnershipProject” (3GPP). CDMA2000 and UMB are described in documents from anorganization named “3rd Generation Partnership Project 2” (3GPP2). Thetechniques described herein may be used for the wireless networks andradio technologies mentioned above as well as other wireless networksand radio technologies, such as a next generation (e.g., 5^(th)Generation (5G)) network.

Embodiments of the present disclosure introduce systems and techniquesto reduce pilot overhead by providing a common reference signal, such asa pilot symbol, that is orthogonal in time and frequency domains toreplace the functionality of existing common reference signals andchannel state information reference signals (CSI-RS) in widebandapplications. Systems and techniques are also introduced forsemi-uniform pilot spacing in both the frequency and time domains.

In an embodiment, pilot sequences are coded with cover codes that areorthogonal in time and frequency domains, resulting in a first set ofpilot symbols that includes multiple pilot symbols in both the time andfrequency domains for transmission at first/second transmit ports and asecond set of pilot symbols that is similarly multi-dimensional in timeand frequency domains for transmission. A receiving entity de-spreadsthe sets of pilot symbols in one or both of the time and frequencydomains to obtain better frequency and time domain estimates with largerpull-in ranges than available with existing pilot structures that canonly be de-spread in one domain or the other. With a common referencesignal that is orthogonal in the time and frequency domains, receivingentities may de-spread the pilot symbols in both domains with increasedresolution for better channel estimates, frequency tracking, timetracking, Doppler estimation, and other measurements useful inestimating channel conditions and making adjustments to improvecommunications.

In another embodiment, sets of pilot symbols are spaced in time fortransmission so that a first pair of pilot symbols is separated by afirst, relatively small time interval. A second pair of pilot symbols isspaced by a second, relatively large time interval from the first pairof pilot symbols. At the receiving end, the first pair of pilot symbolscan be used to generate a coarse estimate of frequency error, and therelatively large time interval between the first and second pairs can beused to generate a fine resolution estimate of frequency error. Thecoarse estimate further refines the fine resolution estimate, e.g., byde-aliasing the fine resolution estimate. The sets of pilot symbols mayalso, or alternatively, be spaced in frequency so that there is a denseset of pilot symbols within a selected frequency band surrounded by, andoverlapping with, a sparse set of pilot symbols throughout the frequencybandwidth, e.g. in the surrounding frequencies as well as the selectedfrequency band. The dense set provides a long time domain window for achannel estimate while the sparse set provides a wideband channelestimate to capture channel estimates across a wide bandwidth, which thedense set can de-alias for better resolution.

FIG. 1 illustrates a wireless communication network 100, in accordancewith various aspects of the present disclosure. The wireless network 100may include a number of base stations 110. A base station 110 mayinclude an evolved Node B (eNodeB) in the LTE context, for example. Abase station may also be referred to as a base transceiver station or anaccess point.

The base stations 110 communicate with user equipments (UEs) 120 asshown. A UE 120 may communicate with a base station 110 via an uplinkand a downlink. The downlink (or forward link) refers to thecommunication link from a base station 110 to a UE 120. The uplink (orreverse link) refers to the communication link from a UE 120 to a basestation 110.

The UEs 120 may be dispersed throughout the wireless network 100, andeach UE 120 may be stationary or mobile. A UE may also be referred to asa terminal, a mobile station, a subscriber unit, etc. A UE 120 may be acellular phone, a smartphone, a personal digital assistant, a wirelessmodem, a laptop computer, a tablet computer, etc. The wirelesscommunication network 100 is one example of a network to which variousaspects of the disclosure apply.

Embodiments of this disclosure are directed to any type of modulationscheme, but orthogonal frequency division multiplexing (OFDM) is used asa representative modulation. OFDM is a multi-carrier modulationtechnique that effectively partitions the overall system bandwidth intomultiple (K) orthogonal frequency subbands. These subbands may also bereferred to as tones, subcarriers, bins, and frequency channels. WithOFDM, each subband is associated with a respective subcarrier that maybe modulated with data. Up to K modulation symbols may be sent on the Ksubbands in each OFDM symbol period.

A pilot symbol may be a symbol known to both the transmitter andreceiver and transmitted in a subband. For an OFDM symbol with Ksubbands, any number and configuration of subbands may be used for pilotsymbols. For example, half of the subbands may be used for pilotsymbols, and the remaining subbands may be used for other purposes, suchas to transmit data symbols or control symbols or the remaining subbandsmay not be used at all. As used herein, a pilot symbol refers to a typeof reference signal as will be recognized by those skilled in therelevant art(s). For simplicity of discussion, reference will be madeherein to “pilot” and “pilot symbol” interchangeably as exemplaryreference signals. An example pilot structure includes a combination ofpilot density and placement (e.g., number of pilot symbols per unit timeor per unit frequency).

The pilot transmission and signaling techniques described herein may beused for a single-input single-output (SISO) system, a single-inputmultiple-output (SIMO) system, a multiple-input single-output (MISO)system, and a multiple-input multiple-output (MIMO) system. Thesetechniques may be used for an OFDM-based system and for othermulti-carrier communication systems. These techniques may also be usedwith various OFDM subband structures.

FIG. 2 is a block diagram illustrating an exemplary transmitter system210 (e.g., a base station 110) and a receiver system 250 (e.g., a UE120) in a MIMO system 200, according to certain aspects of the presentdisclosure. At the transmitter system 210, traffic data for a number ofdata streams is provided from a data source 212 to a transmit (TX) dataprocessor 214.

In a downlink transmission, for example, each data stream is transmittedover a respective transmit antenna. TX data processor 214 formats,codes, and interleaves the traffic data for each data stream based on aparticular coding scheme selected for that data stream to provide codeddata.

The coded data for each data stream may be multiplexed with pilot datausing OFDM techniques. The pilot data, e.g., a pilot sequence, istypically a known data pattern that is processed in a known manner andmay be used at the receiver system to estimate the channel response orother channel parameters. Pilot data may be formatted into pilotsymbols. The number of pilot symbols and placement of pilot symbolswithin an OFDM symbol may be determined by instructions performed byprocessor 230.

The multiplexed pilot and coded data for each data stream is thenmodulated (i.e., symbol mapped) based on a particular modulation scheme(e.g., BPSK, QSPK, M-PSK, or M-QAM) selected for that data stream toprovide modulation symbols. The data rate, coding, and modulation foreach data stream may be determined by instructions performed byprocessor 230. The number of pilot symbols and placement of the pilotsymbols in each frame may also be determined by instructions performedby processor 230.

The processor 230 may be implemented using a general-purpose processor,a digital signal processor (DSP), an application specific integratedcircuit (ASIC), a field programmable gate array (FPGA) or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or any combination thereof designed to perform thefunctions described herein. The processor 230 may also be implemented asa combination of computing devices, e.g., a combination of a DSP and amicroprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration.

The transmitter system 210 further includes a memory 232. The memory 232may be any electronic component capable of storing information and/orinstructions. For example, the memory 250 may include random accessmemory (RAM), read-only memory (ROM), flash memory devices in RAM,optical storage media, erasable programmable read-only memory (EPROM),registers, or combinations thereof. In an embodiment, the memory 232includes a non-transitory computer-readable medium.

Instructions or code may be stored in the memory 232 that are executableby the processor 230. The terms “instructions” and “code” should beinterpreted broadly to include any type of computer-readablestatement(s). For example, the terms “instructions” and “code” may referto one or more programs, routines, sub-routines, functions, procedures,etc. “Instructions” and “code” may include a single computer-readablestatement or many computer-readable statements.

The modulation symbols for all data streams are then provided to a TXMIMO processor 220, that may further process the modulation symbols(e.g., for OFDM). TX MIMO processor 220 then provides N_(T) modulationsymbol streams to N_(T) transmitters (TMTR) 222 _(a) through 222 _(t).In some embodiments, TX MIMO processor 220 applies beamforming weightsto the symbols of the data streams and to the antenna from which thesymbol is being transmitted. The transmitter system 210 includesembodiments having only one antenna or having multiple antennas.

Each transmitter 222 receives and processes a respective symbol streamto provide one or more analog signals, and further conditions (e.g.,amplifies, filters, and upconverts) the analog signals to provide amodulated signal suitable for transmission over the MIMO channel. N_(T)modulated signals from transmitters 222 a through 222 t are thentransmitted from N_(T) antennas 224 _(a) through 224 _(t), respectively.The techniques described herein apply also to systems with only onetransmit antenna. Transmission using one antenna is simpler than themulti-antenna scenario. For example, there may be no need for TX MIMOprocessor 220 in a single antenna scenario.

At receiver system 250, the transmitted modulated signals are receivedby N_(R) antennas 252 _(a) through 252 _(r) and the received signal fromeach antenna 252 is provided to a respective receiver (RCVR) 254 _(a)through 254 _(r). Each receiver 254 conditions (e.g., filters,amplifies, and downconverts) a respective received signal, digitizes theconditioned signal to provide samples, and further processes the samplesto provide a corresponding “received” symbol stream. The techniquesdescribed herein also apply to embodiments of receiver system 250 havingonly one antenna 252.

An RX data processor 260 then receives and processes the N_(R) receivedsymbol streams from receivers 254 _(a) through 254 _(r) based on aparticular receiver processing technique to provide N_(T) detectedsymbol streams. The RX data processor 260 then demodulates,deinterleaves, and decodes as necessary each detected symbol stream torecover the traffic data for the data stream. The processing by RX dataprocessor 260 can be complementary to that performed by TX MIMOprocessor 220 and TX data processor 214 at transmitter system 210.

Information provided by the RX data processor 260 allows the processor270 to generate reports such as channel state information (CSI) andother information to provide to the TX Data Processor 238. Processor 270formulates a reverse link message comprising the CSI and/or pilotrequest to transmit to the transmitter system.

The processor 270 may be implemented using a general-purpose processor,a digital signal processor (DSP), an application specific integratedcircuit (ASIC), a field programmable gate array (FPGA) or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or any combination thereof designed to perform thefunctions described herein. The processor 270 may also be implemented asa combination of computing devices, e.g., a combination of a DSP and amicroprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration.

The reverse link message may comprise various types of informationregarding the communication link and/or the received data stream. Thereverse link message can be processed by a TX data processor 238,modulated by a TX MIMO processor 280, conditioned by transmitters 254_(a) through 254 _(r), and transmitted back to transmitter system 210.As shown, the TX data processor 238 may also receive traffic data for anumber of data streams from a data source 236.

At transmitter system 210, the modulated signals from receiver system250 are received by antennas 224, conditioned by receivers 222,demodulated by a demodulator 240, and processed by a RX data processor242 to extract the reverse link message transmitted by the receiversystem 250.

FIGS. 3A-3B illustrate downlink frame structures for a pilot signalmultiplexing design using multiple transmit ports in a wirelesscommunication network (e.g., the wireless communication network shown inFIG. 1), in accordance with various aspects of the present disclosure.The transmission timeline for the downlink may be partitioned into unitsof transmission time intervals (TTIs) (not shown in FIG. 3A or 3B). ATTI may be related to the size of the data blocks passed from the highernetwork layers to the radio link layer. In some embodiments, theduration of symbols, such as OFDM symbols, is fixed, and there are apredetermined number of symbol periods during each TTI. For example,each TTI may be any number of symbol periods, such as 8, 10, or 12symbol periods, as examples. In an example, each TTI may include eightOFDM symbol periods, and the symbol periods assigned indices fortracking purposes. A transmission during a TTI may be referred to as aframe, a subframe, or a data block. An OFDM symbol period is an exampletime slot.

A number of resource elements may be available in each OFDM symbolperiod. Each resource element may cover one subcarrier in one symbolperiod and may be used to send one modulation symbol, which may be areal or complex value.

As shown in each of FIGS. 3A-3B, there are 11 resource elements per OFDMsymbol as an illustrative example. The resource elements are assignedindices 0 through 11 as shown. Pilot symbols are transmitted in thedesignated resource elements and are denoted in FIGS. 3A and 3B aseither “+” or “−” as discussed in more detail below. The remainingresource elements are available for other types of symbols, such as datasymbols or control symbols, or are simply unused or muted. There areother symbol periods within the TTI, as will be recognized, which arenot shown in FIGS. 3A-3B for ease of illustration.

The pilot structures in FIGS. 3A-3B represent a signal formattransmitted from at least two antenna ports. For example, in a MIMOsystem, the illustrated frame structures are transmitted from two ports,the ports 306 and 312. Each antenna from among a plurality of antennasmay transmit the same or a different pilot structure. In one embodiment,the illustrated pilot structures are received by receive antennas, andmay be part of a composite signal that is a sum of signals from aplurality of antennas at the receiving entity (e.g., a common referencesignal).

FIG. 3A illustrates a pilot structure for the two transmission ports 306and 312 according to an exemplary embodiment. As will be recognized,more pilot structures may be transmitted from additional transmissionports according to embodiments of the present disclosure. For purposesof simplicity, the following discussion will focus on two transmissionports as exemplary. According to the embodiment in FIG. 3A, pilotsymbols are transmitted in OFDM symbol periods 0 and 1 in a given TTI(shown as the two columns for each of ports 306 and 312, respectively).Within periods 0 and 1, pilot symbols are transmitted in resourceelements 0, 2, 4, 6, 8, and 10. As will be recognized, more or fewerresource elements than those shown may alternatively be used oravailable. Further, the pilot symbols may be transmitted in differentperiods of each TTI, e.g. at the start or at the end of each TTI to namesome examples. The pilot symbols may be used for channel estimation,frequency tracking, and time tracking to name a few examples, forcoherent demodulation of the physical channel.

Focusing first on port 306 of FIG. 3A, a first pilot sequence 302 may beprovided, for example to the TX data processor 214 discussed above withrespect to FIG. 2. The first pilot sequence 302 is multiplexed bymultiplying with orthogonal cover code 304. The orthogonal cover code304 may be, for example, a Walsh-Hadamard cover code. In the examplegiven, the orthogonal cover code 304 is a codeword of length 4 (e.g., acode taken from a row of a 4×4 Walsh matrix), illustrated in FIG. 3A as[1 −1 −1 1] in a 2×2 matrix. Other lengths would be possible as well, aswill be recognized by those skilled in the relevant art(s). In theexample of FIG. 3A, the orthogonal cover code 304 is applied so as toprovide multi-dimensionality in the time and frequency domains. Forexample, the first two values, here 1 and −1 shown in the top row of the2×2 matrix, are applied over two symbol periods for a first subcarrier0. The last two values, here −1 and 1 shown in the bottom row of the 2×2matrix, are applied over a second subcarrier 2 in the same two symbolperiods.

As shown in FIG. 3A, use of the orthogonal cover code 304 with the firstpilot sequence 302 results in the pilot symbol structure shown in thecolumns for port 306. Looking at group 314 as an example, the pilotsymbol in period 0 of resource element 0 has a positive value (e.g.,+1), while the pilot symbol in period 1 of resource element 0 has anegative value (e.g., −1). As a further example, the pilot symbol inperiod 0 of resource element 2 has a negative value (e.g., −1) while thepilot symbol in period 1 of resource element 2 has a positive value(e.g., +1). This is a result of the particular orthogonal cover code 304as discussed above. This pattern is then repeated in the other groupsfor the first port 306 across additional subcarriers, as can be seen inFIG. 3A. After spreading, the spread sequence is supplied to the firstport 306 for transmission.

A second pilot sequence 308 is also provided, for example to the TX dataprocessor 214. In the embodiment of FIG. 3A, the pilot sequences 302 and308 are not the same, e.g. the pilot sequences 302 and 308 are differentfrom each other. The second pilot sequence 308 is multiplexed bymultiplying with orthogonal cover code 310. The orthogonal cover code310 may also be a Walsh-Hadamard cover code. As shown, the orthogonalcover code 310 has a sequence [1 1 1 1] illustrated in a 2×2 matrix. Aswill be recognized, orthogonal cover code 310 is orthogonal toorthogonal cover code 304. The codes may be, for example, taken fromdifferent rows of a Walsh matrix where the rows are mutually orthogonal.In the example of FIG. 3A, the orthogonal cover code 310 is applied soas to provide multi-dimensionality in the time and frequency domains aswell. For example, the first two values, here 1 and 1 shown in the toprow of the 2×2 matrix, are applied over two symbol periods for a firstsubcarrier 0. The last two values, here 1 and 1 shown in the bottom rowof the 2×2 matrix, are applied over a second subcarrier 2 in the sametwo symbol periods. In some embodiments, the symbol periods at the firstand second ports 306, 312 are the same.

As shown in FIG. 3A, use of the orthogonal cover code 310 with thesecond pilot sequence 308 results in the pilot symbol structure shown inthe columns for the second port 312. Looking at group 316 as an example,the pilot symbols in periods 0 and 1 of resource elements 0 and 2 eachhave positive values (e.g., +1). Turning to group 318, the pilot symbolsin periods 0 and 1 of resource elements 4 and 6 each have negativevalues (e.g., −1). Since the same orthogonal cover code 310 is appliedto each of groups 316 and 318, the values for the pilot symbols in thegroup 318 illustrate that the second pilot sequence 308 varies betweenthe two groups 316 and 318. For example, in FIG. 3A it can be seen thatthe first pilot sequence 302 can include a series of 1 values (e.g., [11 1 1 . . . n]). In contrast, the second pilot sequence 308 can includeboth 1 and −1 values (e.g., [1 1 1 1 −1 −1 −1 −1 . . . n]). This patternof the second pilot sequence 308 can be repeated in the other groups forthe second port 312 across additional subcarriers, as can be seen inFIG. 3A. After spreading by the orthogonal cover code 310, the spreadsequence can be supplied to the second port 312 for transmission.

The spread sequences are then transmitted by multiple antennas from thefirst and second ports 306 and 312. In an embodiment, the spreadsequences are additionally scrambled in the frequency domain beforetransmission, using either the same or different scrambling codes ateach port. As can be seen, each of ports 306 and 312 transmit theirrespective pilot symbols in the same periods (e.g., 0 and 1) of the sameresource elements (e.g., 0 and 2, etc.), and thus may result in amultiplexed, composite pilot symbol pair consisting of a combination ofthe pilot symbols generated at port 306 and the pilot symbols generatedat port 312. The above patterns multiplexed at the first and secondports 306 and 312 are used for a common reference signal. Because itutilizes two resource elements in both the time and frequency domains,embodiments of the present disclosure provide a substitute referencesignal for both the common reference signal and the CSI-RS. In thisregard, the conventional common RS is frequency division multiplexed(FDM) so that the symbols at two transmit ports are not overlapping witheach other, while the CSI-RS is spread over the time domain but not thefrequency domain.

In contrast, the signals transmitted from ports 306 and 312 areorthogonal to each other in both the time and frequency domains, so thatdata may be recovered in both the frequency and time domains. Inembodiments of the present disclosure, pilot overhead may therefore bereduced. The signals transmitted from ports 306 and 312 may be used forpurposes other than data demodulation. This is in contrast to referencesignals used for data demodulation, such as the demodulation referencesignal (DMRS). DMRS is a reference signal that must be sent along withactual data and is specific to individual users. In other words, theDMRS is only present when a UE has data to send; otherwise, the DMRS isnot sent (in the uplink or downlink) and therefore cannot be used fortime or frequency correction (because it is absent), or duringconnection setup, etc. In contrast, the modified common reference signalaccording to embodiments of the present disclosure may be used for bothfrequency and time tracking correction for all UEs (regardless ofconnection status), since it is transmitted whether there is datapresent as well or not (e.g., whether UEs are idle or connected). Thecommon reference signals transmitted from ports 306 and 312 may be used,instead, for channel state feedback, tracking loops, and control channeldemodulation, to name a few examples, These purposes do not have tosupport a very high data throughput at high signal-to-noise ratio. Forexample, the SNR ceiling in embodiments of the present disclosure may beset to be greater than 10 dB.

On the receiving end, a receiving entity such as a UE 250 receives thetwo spread sequences of the composite signal at corresponding receivers,such as RCVR 254 a and 254 b. The RCVRs 254 condition their respectivereceived signals, digitize the conditioned signals, and produce receivedsymbol streams. A processor, such as the RX data processor 260 of FIG.2, receives the symbol streams from the receivers and de-spreads thedetected symbol streams and/or performs one or more channel estimationschemes to the received symbol streams. In embodiments of the presentdisclosure, by providing multiple symbols in both frequency and timedomains (and that are orthogonal in each), the received symbol streamsmay be de-spread in one or both of the time and frequency domains orperform channel estimation jointly over both time and frequency.

De-spreading in the time domain doubles the channel estimation windowover what would be conventionally possible from reference signals spreadin just one domain, as well as increases a pull-in range for a timetracking loop. Thus, if the processor de-spreads the received symbolstreams in the time domain, one pilot observation (e.g., one pilotsymbol) will be recovered in the time domain but a denser pilot will berecovered in the frequency domain than would conventionally occur. Withthis denser pilot and increased window, better estimates may be made ofthe channel in the frequency domain, for example used in estimating longchannel delay spread.

De-spreading in the frequency domain supports a wide pull-in range for afrequency tracking loop, for example over 18 kHz. If the processorde-spreads the received symbol streams in the frequency domain, twopilot observations (e.g., two pilot symbols) in the time domain will berecovered while a sparser pilot (e.g., sparser than what is recovered byde-spreading in the time domain mentioned above) is recovered in thefrequency domain. With two pilot observations, frequency tracking may beperformed. In an embodiment, the processor may de-spread the receivedsymbol streams in both the time and frequency domains. As will berecognized, the received symbol streams may be de-spread in just one ofthe two domains in the alternative, or a channel estimation may bederived jointly from the two-dimensional orthogonal cover coded pilots.

FIG. 3B illustrates a pilot structure for the two transmission ports 306and 312 according to an alternative exemplary embodiment. For purposesof simplicity of discussion, focus will be on aspects that are differentfrom what was discussed above with respect to FIG. 3A. In FIG. 3B, thesame pilot sequence 320 is provided for eventual transmission at each ofports 306 and 312 (instead of different pilot sequences 302, 308 in FIG.3A). The pilot sequence 320 is multiplexed by multiplying withorthogonal cover code 304 as discussed above with respect to FIG. 3A,resulting in the pilot symbol structure shown in the columns for port306.

A copy of the pilot sequence 320 is also multiplexed by multiplying withorthogonal cover code 310 as discussed above with respect to FIG. 3A,resulting in the pilot symbol structure shown in the columns for port312. As can be seen, since the orthogonal cover code 310 is [1 1 1 1],and the same pilot sequence 320 is provided to each port, the resultingpilot symbols have the same values as the initial pilot sequence 320.Looking at group 316 as an example, the pilot symbols in periods 0 and 1of resource elements 0 and 2 each have positive values (e.g., +1).Turning to group 318, the pilot symbols in periods 0 and 1 of resourceelements 4 and 6 also each have positive values. It can be seen that thepilot sequence 320 can include a series of 1 values (e.g., [1 1 1 1 . .. n]). This pattern can be repeated in the other groups for the secondport 312 across additional subcarriers, as can be seen in FIG. 3B.

The spread sequences are then transmitted from the first and secondports 306 and 312. In an embodiment, the spread sequence at the firstport 306 is additionally scrambled in the frequency domain beforetransmission while the spread sequence at the second port 312 is not. Inthis scenario, an inverse fast Fourier transform (IFFT) of the spreadsequence at the first port 306 corresponds to a time domain shift ofhalf of the channel impulse response length for the first port 306. Inan alternative embodiment, if a random scrambling sequence is applied tothe spread sequence at the second port 312, when estimating the channelimpulse response for one port the channel from the other port may becomethe noise floor.

The embodiment in FIG. 3B may be suitable in situations where there isrelatively low channel delay-spread. In such situations, the receivedsymbol stream transmitted from the second port 312 may appear as analiased signal to the received symbol stream from the first port 306,which may be processed and addressed during channel estimation.

The embodiment in FIG. 3A may be suitable when the channel delay-spreadis not low. Either port's transmission may appear as interference to thesignal transmitted from the other port, which may be suitably processedto remove the noise as will be recognized by those skilled in therelevant art(s). The decision to use either the embodiment of FIG. 3A orthe embodiment of FIG. 3B may be predetermined, e.g. at the time ofnetwork deployment, or dynamically determined for example based onreceiving entity preferences (e.g., a UE's preferences or request).

The above discussion has focused on two transmit ports for ease ofillustration. As will be recognized, in a MIMO system there could bemore than two transmit ports/antennas. In an embodiment, the samepattern may be reproduced at other port/antenna pairs. Alternatively,different patterns may be produced at the other port/antenna pairs usingdifferent cover codes that maintain orthogonality in the pair, as wellas to the first two ports.

FIG. 4 is a protocol diagram illustrating some signaling aspects betweena transmitting entity, such as a base station 110, and a receivingentity, such as a UE 120, for supporting common reference signalsmultiplexed using orthogonal cover codes and multiple transmit ports inaccordance with various aspects of the present disclosure.

In action 402, separate cover codes are applied to pilot sequences foreach of two transmit ports at the base station 110. These cover codes,with additional processing, result in pilot symbols for each port thatare orthogonal to each other in both the time and frequency domains. Forexample, separate Walsh codes (e.g., from different rows of a Walshmatrix) may be applied to each pilot sequence to result in two symbolsin the time domain per subband, and two subbands per group. The pilotsequences may be different for each port or the same, as discussed abovefor FIGS. 3A and 3B, respectively.

In action 404, the pilot symbols are transmitted from both transmitports of the base station 110, such as ports 306 and 312 of FIGS. 3A and3B above, e.g. using the same resource elements resulting in a compositepilot symbol pair.

In action 406, the UE 120 receives the pilot symbols of the compositepilot symbol pair with corresponding receivers and de-spreads thereceived pilot symbols in the time and/or frequency domains. This ispossible because the base station transmitted multiple pilot symbols inboth time and frequency domains from each transmit port. De-spreading inthe time domain results in a denser spread of pilot symbols in thefrequency domain than would otherwise be available (for example, toestimate long channel delay spread), and de-spreading in the frequencydomain results in multiple pilot symbols recovered in the time domain.

In action 408, the UE 120 performs channel estimation, frequencytracking, and/or time tracking as a result of the de-spreading. Forexample, the UE 120 performs channel estimation and/or a time trackingloop when the received pilot symbols are de-spread over time. The UE 120updates a frequency tracking loop with a wide pull-in range when thereceived pilot symbols are de-spread over frequency. In an embodiment,the UE 120 de-spreads over both time and frequency to take advantage ofthe larger time and frequency pull-in ranges and denser pilot structurein the time or frequency domains (de-spread over frequency and time,respectively).

In action 410, the UE 120 responds to the base station 110. For example,the UE 120 may modify one or more parameters in response to informationderived from channel estimation, time tracking, and/or frequencytracking to name a few examples and use the modified parameters inresponding to the base station 110. Further, the UE 120 may includeinformation about the quality of the channels used as part of theresponse back to the base station 110.

The base station 110 may also measure characteristics of the channelsused, such as Doppler spread, channel delay spread, interferencemeasurements, and/or signal-to-noise-plus-interference ratios. Forexample, the base station 110 may use uplink measurements to changedownlink data structures, including the pilot sequences used for pilotsymbol formation.

FIG. 5A is a flowchart illustrating an exemplary method 500 forgenerating and multiplexing a common reference signal using multipletransmit ports in accordance with various aspects of the presentdisclosure. The method 500 may be implemented in a base station 110 thatis in communication with one or more UEs 120. The method 500 may beimplemented in the transmitter system 210 of FIG. 2 above. Instructionsor code may be stored in the memory 232 that are executable by theprocessor 230 and/or the TX data processor 214 in the transmitter system210 to implement the method 500.

At step 502, the processor receives pilot sequences for two transmitports. For example, the TX data processor 214 may receive the pilotsequences from the data source 212 or from some other source. In oneembodiment, the processor receives two separate pilot sequences, one foreach transmit port. In an alternative embodiment, the processor receivesthe same pilot sequence that will be multiplexed with different covercodes for the different transmit ports.

At step 504, the processor applies a first cover code to a first pilotsequence. As a result of applying the first cover code to the firstpilot sequence, multiple pilot symbols are produced for the firsttransmit port, for example two pilot symbols in two different symbolperiods in both of two different sub-bands in a TTI.

At step 506, the processor applies a second cover code to a second pilotsequence. The first and second cover codes are orthogonal to each otherin both the time and frequency domains. As a result of applying thesecond cover code to the second pilot sequence, multiple pilot symbolsare produced for the second transmit port, for example two pilot symbolsin two different symbol periods in both of two different sub-bands in aTTI.

At step 508, the processor provides the pilot symbols to theirrespective transmit ports, for example to transmitters 222 _(a) and 222_(b), and the pilot symbols are transmitted as data streams over theirrespective transmit antennas. As the pilot symbols at the respectivetransmit ports may be transmitted using the same resource elements inthe same time slots, they may constitute a composite pilot symbol pairduring transmission over the air.

FIG. 5B is a flowchart illustrating an exemplary method 520 forutilizing a common reference signal received at multiple receiver portsin accordance with various aspects of the present disclosure. The method520 may be implemented in a UE 120 that is in communication with a basestation 110. The method 520 may be implemented in the receiver system250 of FIG. 2 above. Instructions or code may be stored in the memory272 that are executable by the processor 270 and/or the RX dataprocessor 260 in the receiver system 250 to implement the method 520.

At step 522, a receiver receives pilot signals transmitted from twotransmit ports at the base station 110 at two receivers, such asreceivers 254 _(a) and 254 _(b). The receivers 254 _(a) and 254 _(b) maycondition their respective received signals, digitize the conditionedsignals, and produce received symbol streams.

At step 524, a processor de-spreads the received symbol streams in thetime domain. De-spreading in the time domain results in one observationin the time domain while increasing the resolution in the frequencydomain, e.g. resulting in a denser collection of pilot symbols thanwould normally be recovered, and in a larger channel estimation windowand pull-in range than conventionally possible.

At step 526, the processor de-spreads the received symbol streams in thefrequency domain. De-spreading in the frequency domain results in atleast two observations in the time domain with a wider frequency pull-inrange than conventionally possible, while resulting in a sparsercollection of pilot symbols in the frequency domain. Although describedin two separate steps, it will be recognized that the acts ofde-spreading in the time and frequency domains may be performed eithersequentially (in either order) or at the same time if there issufficient processing power/availability. Further, the processor mayde-spread in only of the time and frequency domains. That is, only oneof steps 524 and 526 is performed in some instances.

At step 528, the observations recovered in the time and frequencydomains from de-spreading over the frequency domain are applied to atime tracking loop and/or to channel estimation.

At step 530, the observations recovered in the time and frequencydomains from de-spreading over the time domain are applied to afrequency tracking loop to address any frequency errors that may havebeen introduced in the channel. Although steps 528 and 530 have beendescribed as two separate steps, it will be recognized that thedifferent loops and estimation may occur in any order, as well as at thesame time.

In addition to increasing the resolution in the frequency and timedomains, there remains opportunity to reduce the overhead associatedwith the different reference signals, such as common reference signalsand CSI reference signals while still maintaining sufficient density toassist in channel estimates and other functions. This may beaccomplished by uniquely spacing the pilot symbols in the time and/orfrequency domains according to embodiments of the present disclosure.

FIG. 6 illustrates a downlink frame structure 600 for a common referencesignal in a semi-uniform time domain arrangement in accordance withvarious aspects of the present disclosure. The downlink frame structure600 includes a small symbol distance used for coarse frequency errordetermination and a large symbol distance used for fine frequency errordetermination. The downlink frame structure 600 is a view from theperspective of the receiver system 250, for example after pilot signalshave been received, conditioned, and decoded.

A first set of pilot symbols are recovered that constitute a coarsefrequency resolution 602. As illustrated, pilot symbols are recovered inthe first two symbol periods on the subcarrier 0. Additional pilotsymbols are recovered in the same two symbol periods on subcarriers 2,4, 6, 8, and 10 as well. Pilot symbols may be recovered in more or fewersubcarriers as well, as will be recognized. The resource element spacesleft blank in FIG. 6's downlink frame structure 600 may be filled withdata symbols and/or various control symbols. As illustrated, the pilotsymbol pair in the downlink frame structure 600 may represent symbolpairs transmitted from any one of antenna ports of the transmittingentity, e.g. port 306 or port 312 of FIGS. 3A/3B, or may representcomposite symbol pairs transmitted from multiple antenna ports at thesame resource elements and times, thus providing orthogonality in theelements that constitute the composite pilot symbol pair (e.g., theconstituent pilot symbol pairs from each transmit port).

This occurs in a first TTI. As will be recognized, the pilot symbols maybe transmitted and received in different symbol periods than the firsttwo. Further, although depicted as in neighboring symbol periods, thesymbol pair in the coarse resolution 602 may alternatively be situated agiven number of symbol periods apart while still within the same TTI.The smaller distance between the symbol pair in the coarse resolution602 provides a wide-range estimate of frequency error introduced in thechannel. It provides a larger pull-in range to provide a coarse estimateof the frequency error. The symbol pair in the coarse resolution 602 maybe spaced in a range less than 100 μs, such as between 15 and 75 μs orbetween 25 and 30 μs, to name just a few examples. In an embodiment, thesymbol pair in the coarse resolution 602 is spread by an orthogonalcover code at the transmitting entity, such as base station 110,according to embodiments discussed above (e.g., at the same time asanother symbol pair(s) at another transmit port using a cover codeorthogonal to the first one).

In a second TTI, a second set of pilot symbols (in the time domain, withmultiple pilot symbols across subcarriers) is recovered. The temporaldistance between the first set of pilot symbols and the second set ofpilot symbols constitutes a fine resolution 604. The fine resolution 604provides a high level of accuracy regarding the amount of frequencyerror introduced in the channel. The symbol pair in the fine resolution604 may be spaced in a range greater than 200 μs, such as around 400 μsor 500 μs, to name just a few examples. The frequency error in thechannel becomes a channel variation over time. Therefore, as thetemporal distance between pilot symbol sets increases, it provides theopportunity to observe how the channel varies over a longer period oftime, resulting in a more accurate estimate of frequency error.

If the frequency error in the channel increases to a point beyond π,aliasing occurs which should be addressed to provide an accurate errorestimate. The coarse resolution 602's wide-range estimate of thefrequency error can be used to de-alias the fine resolution 604. Afterde-aliasing, the frequency estimate can be used in a frequency trackingloop.

The downlink frame structure 600 continues this pattern, illustrated inFIG. 6 with the third set of pilot symbols (with symbol pairs at each oftwo transmit ports) in a third TTI that constitutes a fine resolution606 (e.g., the temporal spacing between the second set of pilot symbolsand the third set of pilot symbols). In an alternative embodiment, thecoarse resolution 602 may be used to de-alias the fine resolution 606instead of the fine resolution 604 (i.e., the first set of pilot symbols602 would not be used as part of a fine resolution estimate, but ratheronly be used for a coarse resolution estimate).

In an embodiment, the periodicity of the sets of pilot symbols is set attime of deployment. Alternatively, the periodicity of the sets of pilotsymbols may be dynamically adjusted during operation, e.g. in responseto an indication or request from one or more receiving entities. Forexample, one or more receiving entities may request that the sets bespaced either closer together or further apart in order to improve fineresolution accuracy.

FIG. 7A is a flowchart illustrating an exemplary method for generatingand transmitting common reference signals in a semi-uniform time domainarrangement in accordance with various aspects of the presentdisclosure. The method 700 may be implemented in a base station 110 thatis in communication with one or more UEs 120. The method 700 may beimplemented in the transmitter system 210 of FIG. 2 above. Instructionsor code may be stored in the memory 232 that are executable by theprocessor 230 and/or the TX data processor 214 in the transmitter system210 to implement the method 700.

At step 702, a transmit port (e.g., associated with one or more physicaltransmitters) receives a set of pilot symbols for transmission to one ormore receiving entities. For example, the transmitters 222 receives theset of pilot symbols, such as a pair of pilot symbols in the time domain(e.g., pilot symbols in adjacent symbol periods) and, in embodiments,spread in the frequency domain across multiple subcarriers asillustrated in FIG. 6. In an embodiment, the transmit port is part of aMIMO system and the transmit port constitutes at least two transmittersto receive pilot symbols after they have been spread by orthogonal covercodes according to embodiments of the present disclosure. Multipletransmit ports may receive a set of pilot symbols for transmission, e.g.at the same times using the same resource elements to result in acomposite pilot symbol pair.

At step 704, the transmit port causes one or more transmitters totransmit the set of pilot symbols so that there is a first time intervalbetween the symbol periods. In an embodiment, the set of pilot symbolsare located in adjacent symbol periods, while in other embodiments thereare one or more symbol periods separating the pilot symbols. Inembodiments where there are differently coded sets of pilot symbols formultiple transmitters, the multiple transmitters transmit theirrespective sets of pilot symbols to corresponding receivers at the oneor more receiving entities.

At step 706, the transmit port receives a second set of pilot symbolsfor transmission to the one or more receiving entities. This second setmay be similar or identical in configuration to the first set, but at adifferent time interval. Multiple transmit ports may again receiverespective second sets of pilot symbols for transmission, e.g. at thesame times using the same resource elements to result in a compositepilot symbol pair again.

At step 708, the transmit port causes one or more transmitters totransmit the second set of pilot symbols to the one or more receivingentities. The second set of pilot symbols is transmitted after a secondtime interval has passed from transmission of the first set of pilotsymbols. The second time interval is greater than the first timeinterval between pilot symbols in the first set of pilot symbols. In anembodiment, the transmit port receives the second set of pilot symbolsafter the second time interval has passed, such that the transmitter mayproceed with transmission without additional delay. In an alternativeembodiment, the transmit port receives the second set of pilot symbolsbefore the second time interval has passed. The transmitter then delaystransmission until the second time interval has passed.

FIG. 7B is a flowchart illustrating an exemplary method 720 forutilizing common reference signals received in a semi-uniform timedomain arrangement in accordance with various aspects of the presentdisclosure. The method 720 may be implemented in a UE 120 that is incommunication with a base station 110. The method 720 may be implementedin the receiver system 250 of FIG. 2 above. Instructions or code may bestored in the memory 272 that are executable by the processor 270 and/orthe RX data processor 260 in the receiver system 250 to implement themethod 720.

At step 722, a receiver receives a first set of pilot symbols from thetransmitting entity. The receiver may be receiver 254 as in FIG. 2above. The first set of pilot symbols may be a pair of pilot symbols inthe time domain (e.g., pilot symbols in adjacent symbol periods) and, inembodiments, spread in the frequency domain across multiple subcarriersas illustrated in FIG. 6. The pair of pilot symbols are spread in thetime domain by a first time interval, which may be very small when thepilot symbols are placed in adjacent symbol periods or small when thereare one or more intervening symbol periods between them. This first timeinterval may be described as providing a coarse resolution, such as thecoarse resolution 602 of FIG. 6 above. In an embodiment, the receiver ispart of a MIMO system and the receiver constitutes at least tworeceivers to receive pilot symbols after they have been spread byorthogonal cover codes and transmitted according to embodiments of thepresent disclosure. In such embodiments, the receiver may process thereceived symbols to recover information in both the time and frequencydomains as discussed above with respect to FIGS. 3A, 3B, 4, 5A, and 5B(e.g., de-spreading in the time and/or frequency domains, etc.).

At step 724, the receiver receives a second set of pilot symbols fromthe transmitting entity. The second set of pilot symbols are receivedafter a second time interval has passed from receiving the first set ofpilot symbols. The second time interval may be described as providing afine resolution, such as the fine resolution 604 of FIG. 6, and islarger than the first time interval. The second set of pilot symbols maybe received during a subsequent TTI to that in which the first set ofpilot symbols was received. The TTIs in which the first and second setsof pilot symbols are received may be adjacent to each other in time orbe separated in time by one or more intervening TTIs.

At step 726, a processor of the receiving entity, such as processor 270and/or the RX data processor 260 in FIG. 2, determines a fine resolutionestimate of frequency error from the channel based on the second timeinterval between the first and second sets of pilot symbols.

At step 728, the processor determines a coarse resolution estimate offrequency error from the channel based on the first time intervalbetween the pilot symbols in the symbol periods of the first set ofpilot symbols. This coarse resolution estimate may be used to de-aliasthe fine resolution estimate of frequency error. In an alternativeembodiment, the coarse resolution may be determined first to set thefrequency error estimate within a wide range and then the fineresolution estimate may be determined within the framework set by thecoarse resolution estimate. With a frequency error estimate that hasbeen de-aliased, the receiving entity may proceed with updating afrequency tracking loop and making adjustments in response to resultsfrom that loop.

FIG. 8 illustrates common reference signal spacing in a semi-uniformfrequency domain arrangement 800 in accordance with various aspects ofthe present disclosure. The semi-uniform frequency domain arrangement800 includes a dense set of pilot symbols within a selected frequencyband, e.g. the centerband, and a sparse set of pilot symbols throughoutthe frequency bandwidth, including within a wideband surrounding theselected frequency band and overlapping with the dense set of pilotsymbols within the selected frequency band. The dense set of pilotsymbols within the selected frequency band provides a better time domainresolution of observations in order to better estimate long delaychannel spread. The sparse set of pilot symbols enables a significantlywideband channel estimate that can capture channel estimates across awide bandwidth, which the dense set can de-alias for better resolution.These estimates are useful for channel energy response estimates andtime tracking loops, to name a couple of examples.

In FIG. 8, there is shown a dense pilot band 802 that occupies theselected frequency band, with a sparse pilot band 804 and a sparse pilotband 806, together, surrounding the dense pilot band 802 on either sideas well as overlapping with the dense set of pilot symbols in the densepilot band 802 (not shown). In an embodiment, the pilot symbols in thedense pilot band 802 may be spaced every 1-2 subcarriers apart within anoverall band of approximately 20 MHz or less. This is just oneexample—it will be recognized that other dense spacings and band sizesare also possible without departing from the scope of the presentdisclosure. The dense pilot band 802 improves the pull-in range in thefrequency domain for a time tracking loop, as well as improves (expands)the channel estimate window size. This is because the dense pilot band802 provides a wide, or coarse, time domain window that provides theimproved pull-in range and the expanded channel estimate window. Withthis dense spacing in the dense pilot band 802, the channel estimatewindow may be extended, for example, to anywhere from 12.5 μs to 25 μsor more.

In an embodiment, the pilot symbols in the sparse pilot bands 804 and806 may be spaced every 5-15 subcarriers apart, with approximately 250pilot symbols total in the two sparse bands. These are just exemplaryvalues, and other spacings in the sparse bands are possible as will berecognized without departing from the scope of the present disclosure.Where the sparse set of pilot symbols overlaps with the dense set in thedense pilot band 802, in one embodiment there may be separate pilotsymbols placed corresponding to the sparse set and neighboring the denseset. In an alternative embodiment, selected ones of the pilot symbols inthe dense set may also serve as pilot symbols in the sparse set. Thesparse set of pilot symbols, e.g. in the sparse pilot bands 804 and 806and overlapping with the dense set in the dense pilot band 802, may beused to provide a more accurate wideband channel estimate in the timedomain that is de-aliased by the estimate from the dense pilot band 802.The transmitting entity may change the subcarriers of the dense pilotband 802 to provide robust performance in a frequency selective channeland so that accuracy may be improved over time.

In addition to providing improved pull-in range, channel estimatewindow, and time tracking loop updating, the sparse set of pilot symbolsin the sparse pilot bands 804/806 and overlapping with the dense set inthe dense pilot band 802 may additionally be used as one or more powerreference signals (power beacon) for an automatic gain control in the UE120, such as when the cell in which the UE 120 is located is unloaded.

FIG. 9A is a flowchart illustrating an exemplary method 900 forgenerating and transmitting common reference signals in a semi-uniformfrequency domain arrangement in accordance with various aspects of thepresent disclosure. The method 900 may be implemented in a base station110 that is in communication with one or more UEs 120. The method 900may be implemented in the transmitter system 210 of FIG. 2 above.Instructions or code may be stored in the memory 232 that are executableby the processor 230 and/or the TX data processor 214 in the transmittersystem 210 to implement the method 900.

At step 902, a processor places a first set of pilot symbols, in thefrequency domain such as in different subcarrier resource elementsillustrated in prior figures, in a dense formation within a selectedfrequency band. The frequency band may have been selected at the time ofdeployment, or may be dynamically selected during operation based on adecision at the base station 110 or per request from one or more UEs120.

At step 904, the processor places a second set of pilot symbols in asparse pilot formation in resource elements surrounding the selectedfrequency band that has the first set of pilot symbols in a denseformation, as well as in the selected frequency band. As a result, thesecond set of pilot symbols overlaps with the first set of pilot symbolsin the region of the selected frequency band.

At step 906, a transmit port receives the first and second sets of pilotsymbols from the processor and transmits the combined set to one or moreUEs 120.

Discussion now turns to FIG. 9B that illustrates a flowchart of anexemplary method 920 for utilizing common reference signals received ina semi-uniform frequency domain arrangement in accordance with variousaspects of the present disclosure. The method 920 may be implemented ina UE 120 that is in communication with a base station 110. The method920 may be implemented in the receiver system 250 of FIG. 2 above.Instructions or code may be stored in the memory 272 that are executableby the processor 270 and/or the RX data processor 260 in the receiversystem 250 to implement the method 920.

At step 922, a receiver of the UE 120 receives a combined set of pilotsymbols transmitted from the base station 110. The combined set of pilotsymbols includes a dense set of pilot symbols in a selected frequencyband surrounded by, and overlapping with in the selected frequency band,a sparse set of pilot symbols.

At step 924, a processor of the UE 120 computes a wide, or coarse, timedomain window channel estimate based on the dense set of pilot symbols,since the dense set of pilot symbols provides an improved pull-in rangeand expanded channel estimate window.

At step 926, the processor computes a wideband channel state informationestimate using the sparse set of pilot symbols that surround (andoverlap in the selected frequency band) the dense set of pilot symbols.

At step 928, the processor de-aliases the wideband CSI estimate based onthe sparse set of pilot symbols based on the coarse estimate based onthe dense set of pilot symbols. In an embodiment, the coarse estimate iscomputed first to set the time domain channel estimate within a widerange and then the wideband CSI estimate based on the sparse set ofpilot symbols may be determined within the framework set by the coarseestimate. With an estimate that has been de-aliased from a wide pull-inrange, the receiving entity may proceed with updating a time trackingloop as well as utilizing the CSI estimate.

Although the semi-uniform time domain and frequency domain arrangementswere discussed with respect to different figures above (e.g., FIGS. 6,7A-7B, 8, and 9A-9B), the time domain and frequency domain arrangementsmay be combined so that sets of pilot symbols may exhibit both so as toharness the benefits of both in a single system. The combination of thetime domain and frequency domain spacing is reflected in FIG. 10.

FIG. 10 illustrates the semi-uniform spacing along both frequency andtime domain axes as labeled in the figure. Pilot symbols 1002 and 1004,shown in FIG. 10 in adjacent symbol periods along the time axis, areexemplary of how the remaining pilot symbols are depicted in FIG. 10. Ascan be seen, along the time axis, the pilot symbols occur in pairs alongany given subcarrier along the frequency axis. As illustrated, the pilotsymbols 1002 and 1004 may constitute a pilot symbol pair. In anembodiment that may be a pilot symbol pair transmitted from a singleantenna port, while in another embodiment that may be a composite pilotsymbol pair as seen over the air (e.g., multiple transmit portstransmitting respective pilot symbol pairs at the same resource elementsand times that are orthogonalized by orthogonal cover codes in the codedomain as discussed in embodiments above).

FIG. 10 illustrates the semi-uniform time domain arrangement with theexemplary coarse resolution 602 and the fine resolution 604. Althoughshown to be in adjacent symbol periods, the pilot symbol pairs mayalternatively have one or more intervening symbol periods between them.Although not labeled, it will be recognized that the other pilot symbolsin FIG. 10 include this same combination of coarse and fine resolutionspacing in time.

FIG. 10 also illustrates the semi-uniform frequency domain arrangementwith the dense pilot band 802 in a band at the center of the plot, andsparse pilot bands 804 and 806 surrounding the dense pilot band 802(with the sparse set of pilot symbols overlapping with the dense set ofpilot symbols in the selected band of the frequency domain). FIG. 10 isfor ease of illustration only—it should be recognized that more or fewerpilot symbols may be included along either or both of the frequency andtime domains than those depicted in FIG. 10.

Information and signals may be represented using any of a variety ofdifferent technologies and techniques. For example, data, instructions,commands, information, signals, bits, symbols, and chips that may bereferenced throughout the above description may be represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, or any combination thereof.

The various illustrative blocks and modules described in connection withthe disclosure herein may be implemented or performed with ageneral-purpose processor, a DSP, an ASIC, an FPGA or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general-purpose processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices (e.g., a combinationof a DSP and a microprocessor, multiple microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration).

The functions described herein may be implemented in hardware, softwareexecuted by a processor, firmware, or any combination thereof. Ifimplemented in software executed by a processor, the functions may bestored on or transmitted over as one or more instructions or code on acomputer-readable medium. Other examples and implementations are withinthe scope of the disclosure and appended claims. For example, due to thenature of software, functions described above can be implemented usingsoftware executed by a processor, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations. Also, as used herein, including in the claims, “or” as usedin a list of items (for example, a list of items prefaced by a phrasesuch as “at least one of” or “one or more of”) indicates an inclusivelist such that, for example, a list of [at least one of A, B, or C]means A or B or C or AB or AC or BC or ABC (i.e., A and B and C).

Embodiments of the present disclosure include a method for wirelesscommunication, comprising processing, by a processor of a wirelesscommunications device, a first pilot sequence to produce a first pilotsymbol pair and a second pilot sequence to produce a second pilot symbolpair; transmitting, by a transmitter, the pilot symbols of the firstpilot symbol pair within a first time interval from each other in a timedomain; and transmitting, by the transmitter, the second pilot symbolpair within a second time interval from the first pilot symbol pair, thesecond time interval being greater than the first time interval.

The method further includes transmitting the first pilot symbol pairduring a first transmission time interval; and transmitting the secondpilot symbol pair during a second transmission time interval, the secondtransmission time interval being subsequent to the first transmissiontime interval. The method further includes wherein the transmittingfurther comprises delivering a second pilot symbol of the first pilotsymbol pair in an adjacent time slot to a first pilot symbol of thefirst pilot symbol pair. The method further includes wherein thetransmitting further comprises transmitting the second pilot symbol pairafter an intervening transmission time interval from the firsttransmission time interval. The method further includes wherein thetransmitter comprises a first transmit port, the method furthercomprising processing, by the processor, a third pilot sequence toproduce a third pilot symbol pair for transmission at the secondtransmit port and a fourth pilot sequence to produce a fourth pilotsymbol pair for transmission at the second transmit port; transmitting,by the second transmit port, the third pilot symbol pair at a sametransmission time interval as the first pilot symbol pair from the firsttransmit port; transmitting, by the second transmit port, the fourthpilot symbol pair at a same transmission time interval as the secondpilot symbol pair from the first transmit port, the first and thirdpilot symbol pairs being spread with cover codes that are orthogonal toeach other in time and frequency domains and first and third pilotsymbol pairs comprising a first common reference signal, and the secondand fourth pilot symbol pairs being spread with cover codes that areorthogonal to each other in time and frequency domains and comprising asecond common reference signal. The method further includes wherein thefirst pilot symbol comprises a plurality of pilot symbols spread acrossa range of frequency subcarriers in a frequency domain, the methodfurther comprising placing a first subset of the plurality of pilotsymbols with a first plurality of subcarriers having a first frequencyspacing from each other and located within a selected frequency band inthe frequency domain; and placing a second subset of the plurality ofpilot symbols with a second plurality of subcarriers having a secondfrequency spacing from each other and surrounding and including theselected frequency band, the second frequency spacing being greater thanthe first frequency spacing. The method further includes wherein thefirst time interval comprises a spacing of 100 microseconds or less; andthe second time interval comprises a spacing of 200 microseconds orgreater.

Embodiments of the present disclosure further include a method forwireless communication, comprising receiving, at a wirelesscommunications device, pilot symbols of a first pilot symbol pair withina first time interval from each other; receiving, at the wirelesscommunications device, pilot symbols of a second pilot symbol pairwithin a second time interval from the first pilot symbol pair, thesecond time interval being greater than the first time interval; andcalculating, by a processor of the wireless communications device, afrequency error of a channel that conveyed the pilot symbol pairs basedon an observation of channel variation during the first and second timeintervals.

The method further includes de-aliasing the calculated frequency errorbased on an observation of channel variation during the first timeinterval. The method further includes receiving the first pilot symbolpair during a first transmission time interval; and receiving the secondpilot symbol pair during a second transmission time interval, the secondtransmission time interval being subsequent to the first transmissiontime interval. The method further includes receiving, at the wirelesscommunications device, a third pilot symbol pair at a same transmissiontime interval as the first pilot symbol pair and a fourth pilot symbolpair at a same transmission time interval as the second pilot symbolpair, the first and third pilot symbol pairs being spread with covercodes that are orthogonal to each other in time and frequency domainsand the first and third pilot symbol pairs comprising a first commonreference signal, and the second and fourth pilot symbol pairs beingspread with cover codes that are orthogonal to each other in time andfrequency domains and the second and fourth pilot symbol pairscomprising a second common reference signal. The method further includesde-spreading the first and third pilot symbol pairs in the frequencydomain to recover at least two pilot observations in the time domain;and updating a frequency tracking loop with the at least two pilotobservations in the time domain. The method further includesde-spreading the first and third pilot symbol pairs in the time domainto recover a pilot observation in the time domain and a dense pilotspread in the frequency domain; and estimating a channel state (withlong delay spread) based on the dense pilot spread recovered in thefrequency domain. The method further includes wherein the first pilotsymbol comprises a plurality of pilot symbols spread across a range ofsubcarriers in a frequency domain, the receiving the first pilot symbolfurther comprising receiving a first subset of the plurality of pilotsymbols placed with a first plurality of subcarriers having a firstfrequency spacing from each other and located within a selectedfrequency band; and receiving a second subset of the plurality of pilotsymbols placed with a second plurality of subcarriers having a secondfrequency spacing from each other and surrounding and including theselected frequency band, wherein the second frequency spacing is greaterthan the first frequency spacing. The method further includescalculating a wideband channel estimate based on the second subset ofthe plurality of pilot symbols having the second frequency spacingsurrounding and including the selected frequency band; calculating acenterband channel estimate based on the first subset of the pluralityof pilot symbols having the first frequency spacing within the selectedfrequency band; and de-aliasing the wideband channel estimate based onthe centerband channel estimate with fine frequency resolution (in orderto estimate long delay spread).

Embodiments of the present disclosure further include a method forwireless communication, comprising processing, by a processor of awireless communications device, a pilot sequence to produce a pluralityof pilot symbols spread across a range of frequency subcarriers in afrequency domain; and transmitting, by a transmitter of the wirelesscommunications device: a first subset of the plurality of pilot symbolswith a first plurality of subcarriers having a first frequency spacingfrom each other and located within a selected frequency band in thefrequency domain, and a second subset of the plurality of pilot symbolswith a second plurality of subcarriers having a second frequency spacingfrom each other and surrounding and including the selected frequencyband, the second frequency spacing being greater than the firstfrequency spacing.

The method further includes transmitting the first and secondpluralities of pilot symbols during a first transmission time interval.The method further includes wherein the first frequency spacingcomprises a spacing of two or fewer subcarriers between pilot symbolsfrom among the first plurality of pilot symbols in the selectedfrequency band; and the second frequency spacing comprises a spacing ofeight or more subcarriers between pilot symbols from among the secondplurality of pilot symbols surrounding and including the selectedfrequency band. The method further includes wherein the pilot sequencecomprises a first pilot sequence and the plurality of pilot symbolscomprises a first plurality of pilot symbols, the method furthercomprising processing, by the processor, a second pilot sequence toproduce a second plurality of pilot symbols; and transmitting, by thetransmitter, pilot symbols of the first plurality of pilot symbolswithin a first time interval from each other and pilot symbols of thesecond plurality of pilot symbols within a second time interval from thefirst plurality of pilot symbols, the second time interval being greaterthan the first time interval. The method further includes wherein thepilot sequence comprises a first pilot sequence and the plurality ofpilot symbols comprises a first plurality of pilot symbols and thetransmitter comprises a first transmit port, the method furthercomprising processing, by the processor, a second pilot sequence toproduce a second plurality of pilot symbols for transmission at a secondtransmit port; and transmitting, by the second transmit port, the secondplurality of pilot symbols at a same transmission time interval as thefirst plurality of pilot symbols from the first transmit port, the firstand second pluralities pilot symbols being spread with cover codes thatare orthogonal to each other in time and frequency domains and the firstand second pluralities of pilot symbols comprising a common referencesignal.

Embodiments of the present disclosure further include a method forwireless communication, comprising receiving, at a wirelesscommunications device, a plurality of pilot symbols spread across aplurality of different subcarriers in a frequency domain, the pluralityof pilot symbols comprising: a first subset of the plurality of pilotsymbols placed with a first plurality of subcarriers having a firstfrequency spacing from each other and located within a selectedfrequency band; and a second subset of the plurality of pilot symbolsplaced with a second plurality of subcarriers having a second frequencyspacing from each other and surrounding and including the selectedfrequency band, wherein the second frequency spacing is greater than thefirst frequency spacing; and calculating, by a processor of the wirelesscommunications device, a wideband channel estimate based on the secondsubset of the plurality of pilot symbols having the second frequencyspacing surrounding and including the selected frequency band.

The method further includes calculating a centerband channel estimatebased on the first subset of the plurality of pilot symbols having thefirst frequency spacing within the selected frequency band. The methodfurther includes de-aliasing the wideband channel estimate based on thecenterband channel estimate. The method further includes wherein theplurality of pilot symbols comprises a first plurality of pilot symbols,the method further comprising receiving, at the wireless communicationsdevice, pilot symbols of the first plurality of pilot symbols within afirst time interval from each other; receiving, at the wirelesscommunications device, the second plurality of pilot symbols within asecond time interval from the first plurality of pilot symbols, thesecond time interval being greater than the first time interval;calculating, by the processor, a frequency error of a channel thatconveyed the plurality of pilot symbols based on an observation ofchannel variation during the second time interval; and de-aliasing thecalculated frequency error based on an observation of channel variationduring the first time interval. The method further includes receiving,at the wireless communications device, a second plurality of pilotsymbols at a same transmission time interval as the first plurality ofpilot symbols, the first and second pluralities pilot symbols beingspread with cover codes that are orthogonal to each other in time andfrequency domains and the first and second pluralities of pilot symbolscomprising a common reference signal. The method further includesde-spreading the first and second pluralities of pilot symbols in thefrequency domain to recover at least two pilot observations in the timedomain; and updating a frequency tracking loop with the at least twopilot observations in the time domain. The method further includesde-spreading the first and second pluralities of pilot symbols in thetime domain to recover a pilot observation in the time domain and adense pilot spread in the frequency domain; and estimating a channelstate with long delay spread based on the dense pilot spread recoveredin the frequency domain.

Embodiments of the present disclosure further include a wirelesscommunications device that comprises at least one receiver configured toreceive a first set of pilot symbols using a number of resource elementsand spread with a first cover code; and receive a second set of pilotsymbols using a second number of resource elements and spread with asecond cover code, the first and second cover codes being orthogonal toeach other in time and frequency domains, the first and second set ofpilot symbols comprising a common reference signal; and a processorconfigured to de-spread the first and second sets of pilot symbols inthe frequency domain to recover at least two pilot observations in thetime domain.

The wireless communications device further includes wherein theprocessor is further configured to de-spread the first and second setsof pilot symbols in the time domain to recover a pilot observation inthe time domain and a dense pilot spread in the frequency domain; andestimate a channel state with long delay spread based on the dense pilotspread recovered in the frequency domain. The wireless communicationsdevice further includes wherein the processor is further configured toupdate a frequency tracking loop with the at least two pilotobservations in the time domain (by de-spreading two pilot ports overthe frequency domain). The wireless communications device furtherincludes wherein the first set of pilot symbols comprises a pair ofpilot symbols received within a first time interval from each other inthe time domain, the at least one receiver further configured to receivea third set of pilot symbols and a fourth set of pilot symbols at thewireless communications device within a second time interval of thefirst and second sets of pilot symbols in the time domain, the secondtime interval being greater than the first time interval. The wirelesscommunications device further includes wherein the processor is furtherconfigured to compute a fine-resolution frequency error of a channelthat conveyed the first and third sets of pilot symbols (and the secondand fourth sets of pilot symbols) based on an observation of channelvariation during the second time interval; and de-alias thefine-resolution frequency error based on an observation of channelvariation during the first time interval. The wireless communicationsdevice further includes wherein the number of resource elements areplaced at different subcarriers in the frequency domain, the at leastone receiver further configured to receive a first subset of the firstset of pilot symbols placed with a first plurality of subcarriers havinga first frequency spacing from each other and located within a selectedfrequency band; and receive a second subset of the first set of pilotsymbols placed with a second plurality of subcarriers having a secondfrequency spacing from each other and surrounding and including theselected frequency band, wherein the second frequency spacing is greaterthan the first frequency spacing.

Embodiments of the present disclosure further include a wirelesscommunications device that comprises a processor configured to process afirst pilot sequence to produce a first pilot symbol pair and a secondpilot sequence to produce a second pilot symbol pair; a transceiverconfigured to transmit the pilot symbols of the first pilot symbol pairwithin a first time interval from each other in a time domain; andtransmit the second pilot symbol pair within a second time interval fromthe first pilot symbol pair, the second time interval being greater thanthe first time interval.

The wireless communications device further includes wherein thetransceiver is further configured to transmit the first pilot symbolpair during a first transmission time interval; and transmit the secondpilot symbol pair during a second transmission time interval, the secondtransmission time interval being subsequent to the first transmissiontime interval. The wireless communications device further compriseswherein the transceiver is further configured to deliver a second pilotsymbol of the first pilot symbol pair in an adjacent time slot to afirst pilot symbol of the first pilot symbol pair. The wirelesscommunications device further includes wherein the transceiver comprisesa first transmit port; and the processor is further configured toprocess third and fourth pilot sequences to produce third and fourthpilot symbol pairs, the transceiver further comprising a second transmitport configured to transmit the third pilot symbol pair at a sametransmission time interval as the first pilot symbol pair from the firsttransmit port and the fourth pilot symbol pair at a same transmissiontime interval as the second pilot symbol pair, the first and third pilotsymbol pairs and the second and fourth pilot symbol pairs beingrespectively spread with cover codes that are orthogonal to each otherin time and frequency domains and first and third pilot symbol pairscomprising a first common reference signal and second and fourth pilotsymbol pairs comprising a second common reference signal. The wirelesscommunications device further includes wherein the first pilot symbolcomprises a plurality of pilot symbols spread across a range offrequency subcarriers in a frequency domain, the transceiver furtherconfigured to place a first subset of the plurality of pilot symbolswith a first plurality of subcarriers having a first frequency spacingfrom each other and located within a selected frequency band in thefrequency domain; and place a second subset of the plurality of pilotsymbols with a second plurality of subcarriers having a second frequencyspacing from each other and surrounding and including the selectedfrequency band, the second frequency spacing being greater than thefirst frequency spacing. The wireless communications device furtherincludes wherein the first time interval comprises a spacing of e.g. 100microseconds or less; and the second time interval comprises a spacingof e.g. 200 microseconds or greater.

Embodiments of the present disclosure further include a wirelesscommunications device comprising at least one receiver configured toreceive pilot symbols of a first pilot symbol pair within a first timeinterval from each other; and receive pilot symbols of a second pilotsymbol pair within a second time interval from the first pilot symbolpair, the second time interval being greater than the first timeinterval; and a processor configured to calculate a frequency error of achannel that conveyed the pilot symbol pairs based on an observation ofchannel variation during the first and second time intervals.

The wireless communications device further includes wherein theprocessor is further configured to de-alias the calculated frequencyerror based on an observation of channel variation during the first timeinterval. The wireless communications device further includes whereinthe at least one receiver is further configured to receive the firstpilot symbol pair during a first transmission time interval; and receivethe second pilot symbol pair during a second transmission time interval,the second transmission time interval being subsequent to the firsttransmission time interval. The wireless communications device furtherincludes wherein the at least one receiver is further configured toreceive a third pilot symbol pair at a same transmission time intervalas the first pilot symbol pair and a fourth pilot symbol pair at a sametransmission time interval as the second pilot symbol pair, the firstand third pilot symbol pairs being spread with cover codes that areorthogonal to each other in time and frequency domains and the secondand fourth pilot symbol pairs being spread with cover codes that areorthogonal in time and frequency domains, and the first and third pilotsymbol pairs comprising a first common reference signal and the secondand fourth pilot symbol pairs comprising a second common referencesignal. The wireless communications device further includes wherein theprocessor is further configured to de-spread the first and third pilotsymbol pairs in the frequency domain to recover at least two pilotobservations in the time domain; and update a frequency tracking loopwith the at least two pilot observations in the time domain. Thewireless communications device further includes wherein the processor isfurther configured to de-spread the first and third pilot symbol pairsin the time domain to recover a pilot observation in the time domain anda dense pilot spread in the frequency domain; and estimate a channelstate based on the dense pilot spread recovered in the frequency domain.The wireless communications device further includes wherein the firstpilot symbol comprises a plurality of pilot symbols spread across arange of subcarriers in the frequency domain, the at least one receiverfurther configured to receive a first subset of the plurality of pilotsymbols placed with a first plurality of subcarriers having a firstfrequency spacing from each other and located within a selectedfrequency band; and receive a second subset of the plurality of pilotsymbols placed with a second plurality of subcarriers having a secondfrequency spacing from each other and surrounding and including theselected frequency band, wherein the second frequency spacing is greaterthan the first frequency spacing. The wireless communications devicefurther includes wherein the processor is further configured tocalculate a wideband channel estimate based on the second subset of theplurality of pilot symbols having the second frequency spacingsurrounding and including the selected frequency band; calculate acenterband channel estimate based on the first subset of the pluralityof pilot symbols having the first frequency spacing within the selectedfrequency band; and de-alias the wideband channel estimate based on thecenterband channel estimate.

Embodiments of the present disclosure further include a wirelesscommunications device comprising a processor configured to process apilot sequence to produce a plurality of pilot symbols spread across arange of frequency subcarriers in a frequency domain; and a transceiverconfigured to transmit a first subset of the plurality of pilot symbolswith a first plurality of subcarriers having a first frequency spacingfrom each other and located within a selected frequency band in thefrequency domain, and transmit a second subset of the plurality of pilotsymbols with a second plurality of subcarriers having a second frequencyspacing from each other and surrounding and including the selectedfrequency band, the second frequency spacing being greater than thefirst frequency spacing.

The wireless communications device further includes wherein thetransceiver is further configured to transmit the first and secondpluralities of pilot symbols during a first transmission time interval.The wireless communications device further includes wherein the firstfrequency spacing comprises a spacing of e.g. two or fewer subcarriersbetween pilot symbols from among the first plurality of pilot symbols inthe selected frequency band; and the second frequency spacing comprisesa spacing of e.g. eight or more subcarriers between pilot symbols fromamong the second plurality of pilot symbols surrounding and includingthe selected frequency band. The wireless communications device furtherincludes wherein the pilot sequence comprises a first pilot sequence andthe plurality of pilot symbols comprises a first plurality of pilotsymbols, the processor is further configured to process a second pilotsequence to produce a second plurality of pilot symbols, and thetransmitter is further configured to transmit pilot symbols of the firstplurality of pilot symbols within a first time interval from each otherand pilot symbols of the second plurality of pilot symbols within asecond time interval from the first plurality of pilot symbols, thesecond time interval being greater than the first time interval. Thewireless communications device further includes wherein the pilotsequence comprises a first pilot sequence and the plurality of pilotsymbols comprises a first plurality of pilot symbols, the transceivercomprises a first transmit port and a second transmit port, theprocessor is further configured to process a second pilot sequence toproduce a second plurality of pilot symbols for transmission at thesecond transmit port, and the second transmit port is configured totransmit the second plurality of pilot symbols at a same transmissiontime interval as the first plurality of pilot symbols from the firsttransmit port, the first and second pluralities pilot symbols beingspread with cover codes that are orthogonal to each other in time andfrequency domains and the first and second pluralities of pilot symbolscomprising a common reference signal.

Embodiments of the present disclosure further include a wirelesscommunications device comprising at least one receiver configured toreceive a plurality of pilot symbols spread across a plurality ofdifferent subcarriers in a frequency domain, the plurality of pilotsymbols comprising a first subset of the plurality of pilot symbolsplaced with a first plurality of subcarriers having a first frequencyspacing from each other and located within a selected frequency band;and a second subset of the plurality of pilot symbols placed with asecond plurality of subcarriers having a second frequency spacing fromeach other and surrounding and including the selected frequency band,wherein the second frequency spacing is greater than the first frequencyspacing; and a processor configured to calculate a wideband channelestimate based on the second subset of the plurality of pilot symbolshaving the second frequency spacing surrounding and including theselected frequency band.

The wireless communications device further includes wherein theprocessor is further configured to calculate a centerband channelestimate based on the first subset of the plurality of pilot symbolshaving the first frequency spacing within the selected frequency band;and de-alias the wideband channel estimate based on the centerbandchannel estimate. The wireless communications device further includeswherein the plurality of pilot symbols comprises a first plurality ofpilot symbols, the at least one receiver further configured to receivepilot symbols of the first plurality of pilot symbols within a firsttime interval from each other; and receive the second plurality of pilotsymbols within a second time interval from the first plurality of pilotsymbols, the second time interval being greater than the first timeinterval; and the processor is further configured to calculate afrequency error of a channel that conveyed the plurality of pilotsymbols based on an observation of channel variation during the secondtime interval and de-alias the calculated frequency error based on anobservation of channel variation during the first time interval. Thewireless communications device further includes wherein the receiver isfurther configured to receive a second plurality of pilot symbols at asame transmission time interval as the first plurality of pilot symbols,the first and second pluralities pilot symbols being spread with covercodes that are orthogonal to each other in time and frequency domainsand the first and second pluralities of pilot symbols comprising acommon reference signal. The wireless communications device furtherincludes wherein the processor is further configured to de-spread thefirst and second pluralities of pilot symbols in the frequency domain torecover at least two pilot observations in the time domain; and update afrequency tracking loop with the at least two pilot observations in thetime domain. The wireless communications device further includes whereinthe processor is further configured to de-spread the first and secondpluralities of pilot symbols in the time domain to recover a pilotobservation in the time domain and a dense pilot spread in the frequencydomain; and estimate a channel state based on the dense pilot spreadrecovered in the frequency domain.

As those of some skill in this art will by now appreciate and dependingon the particular application at hand, many modifications, substitutionsand variations can be made in and to the materials, apparatus,configurations and methods of use of the devices of the presentdisclosure without departing from the spirit and scope thereof. In lightof this, the scope of the present disclosure should not be limited tothat of the particular embodiments illustrated and described herein, asthey are merely by way of some examples thereof, but rather, should befully commensurate with that of the claims appended hereafter and theirfunctional equivalents.

What is claimed is:
 1. A method for wireless communication, comprising: applying, by a processor of a wireless communications device, a first cover code to a first pilot sequence to produce a first set of pilot symbols and a second cover code to a second pilot sequence to produce a second set of pilot symbols, wherein the first cover code and the second cover code are orthogonal to each other in time and frequency domains; delivering the first set of pilot symbols using a first number of resource elements to a first transmit port; delivering the second set of pilot symbols using a second number of resource elements to a second transmit port; transmitting, from the first transmit port, the first set of pilot symbols; and transmitting, from the second transmit port, the second set of pilot symbols, the first and second transmitted sets of pilot symbols comprising a modified common reference signal as a substitute for a common reference signal and a channel state information reference signal.
 2. The method of claim 1, wherein the applying comprises: spreading the first pilot sequence with a first Walsh code comprising the first cover code; and spreading the second pilot sequence with a second Walsh code comprising the second cover code.
 3. The method of claim 1, further comprising: scrambling the first and second sets of pilot symbols with a scrambling code in the frequency domain.
 4. The method of claim 1, wherein: the first pilot sequence is different from the second pilot sequence, and the first and second number of resource elements are the same.
 5. The method of claim 1, wherein: the first pilot sequence is the same as the second pilot sequence, and the first and second number of resource elements are the same.
 6. The method of claim 1, wherein the first set of pilot symbols comprises a pair of pilot symbols in the time domain, the transmitting further comprising: transmitting the pair of pilot symbols within a first time interval of each other in the time domain; transmitting a third set of pilot symbols via the first transmit port within a second time interval of the pair of pilot symbols; and transmitting a fourth set of pilot symbols via the second transmit port within the second time interval, the first time interval being less than the second time interval and the third and fourth sets of pilot symbols comprising a second modified common reference signal.
 7. The method of claim 1, wherein the first and second numbers of resource elements are placed at different subcarriers in the frequency domain, the transmitting further comprising: placing a first subset of the first set of pilot symbols with a first plurality of subcarriers having a first frequency spacing from each other and located within a selected frequency band in the frequency domain; and placing a second subset of the first set of pilot symbols with a second plurality of subcarriers having a second frequency spacing from each other and surrounding and including the selected frequency band, the second frequency spacing being greater than the first frequency spacing.
 8. A wireless communications device, comprising: a processor configured to: apply a first cover code to a first pilot sequence to produce a first set of pilot symbols and a second cover code to a second pilot sequence to produce a second set of pilot symbols, wherein the first cover code and the second cover code are orthogonal to each other in time and frequency domains; deliver the first set of pilot symbols using a first number of resource elements for transmission; and deliver the second set of pilot symbols using a second number of resource elements for transmission; and a transceiver comprising a first transmit port and a second transmit port, the first transmit port configured to transmit the first set of pilot symbols and the second transmit port configured to transmit the second set of pilot symbols, the first and second transmitted sets of pilot symbols comprising a modified common reference signal as a substitute for a common reference signal and a channel state information reference signal.
 9. The wireless communications device of claim 8, wherein the processor is further configured to: spread the first pilot sequence with a first Walsh code comprising the first cover code; and spread the second pilot sequence with a second Walsh code comprising the second cover code.
 10. The wireless communications device of claim 8, wherein the transceiver is further configured to scramble the first and second sets of pilot symbols with respective scrambling codes in the frequency domain.
 11. The wireless communications device of claim 8, wherein the first pilot sequence is different from the second pilot sequence and the first and second number of resource elements are the same.
 12. The wireless communications device of claim 8, wherein the first pilot sequence is the same as the second pilot sequence and the first and second number of resource elements are the same.
 13. The wireless communications device of claim 8, wherein the first set of pilot symbols comprises a pair of pilot symbols in the time domain, the transceiver further configured to: transmit the pair of pilot symbols within a first time interval of each other in the time domain by the first transmit port; transmit a third set of pilot symbols within a second time interval of the pair of pilot symbols; and transmit a fourth set of pilot symbols within the second time interval by the second transmit port, the first time interval being less than the second time interval and the third and fourth sets of pilot symbols comprising a second modified common reference signal.
 14. The wireless communications device of claim 8, wherein the first and second numbers of resource elements are placed at different subcarriers in the frequency domain, the first transmit port of the transceiver further configured to: place a first subset of the first set of pilot symbols with a first plurality of subcarriers having a first frequency spacing from each other and located within a selected frequency band in the frequency domain; and place a second subset of the first set of pilot symbols with a second plurality of subcarriers having a second frequency spacing from each other and surrounding and including the selected frequency band, the second frequency spacing being greater than the first frequency spacing.
 15. A non-transitory computer-readable medium having program code recorded thereon, the program code comprising: code for causing a wireless communications device to apply a first cover code to a first pilot sequence to produce a first set of pilot symbols and a second cover code to a second pilot sequence to produce a second set of pilot symbols, wherein the first cover code and the second cover code are orthogonal to each other in time and frequency domains; code for causing the wireless communications device to deliver the first set of pilot symbols using a first number of resource elements to a first transmit port; code for causing the wireless communications device to deliver the second set of pilot symbols using a second number of resource elements to a second transmit port; code for causing the wireless communications device to transmit, from the first transmit port, the first set of pilot symbols; and code for causing the wireless communications device to transmit, from the second transmit port, the second set of pilot symbols, the first and second sets of pilot symbols comprising a modified common reference signal as a substitute for a common reference signal and a channel state information reference signal.
 16. The non-transitory computer-readable medium of claim 15, further comprising: code for causing the wireless communications device to scramble the first and second sets of pilot symbols with a scrambling code in the frequency domain.
 17. The non-transitory computer-readable medium of claim 15, wherein the first pilot sequence is different from the second pilot sequence and the first and second number of resource elements are the same.
 18. The non-transitory computer-readable medium of claim 15, wherein the first pilot sequence is the same as the second pilot sequence and the first and second number of resource elements are the same.
 19. The non-transitory computer-readable medium of claim 15, wherein the first set of pilot symbols comprises a pair of pilot symbols in the time domain, further comprising: code for causing the wireless communications device to transmit the pair of pilot symbols within a first time interval of each other in the time domain; and code for causing the wireless communications device to transmit a third set of pilot symbols via the first transmit port within a second time interval of the pair of pilot symbols, the first time interval being less than the second time interval.
 20. The non-transitory computer-readable medium of claim 15, wherein the first and second numbers of resource elements are placed at different subcarriers in the frequency domain, further comprising: code for causing the wireless communications device to place a first subset of the first set of pilot symbols with a first plurality of subcarriers having a first frequency spacing from each other and located within a selected frequency band in the frequency domain; and code for causing the wireless communications device to place a second subset of the first set of pilot symbols with a second plurality of subcarriers having a second frequency spacing from each other and surrounding and including the selected frequency band, the second frequency spacing being greater than the first frequency spacing.
 21. A method for wireless communication, comprising: receiving, at a wireless communications device, a first set of pilot symbols using a first number of resource elements and spread with a first cover code; receiving, at the wireless communications device, a second set of pilot symbols using a second number of resource elements and spread with a second cover code, the first and second cover codes being orthogonal to each other in time and frequency domains, the first and second sets of pilot symbols comprising a modified common reference signal as a substitute for a common reference signal and a channel state information reference signal; and de-spreading the first and second sets of pilot symbols in the frequency domain to recover at least two pilot observations in the time domain.
 22. The method of claim 21, further comprising: de-spreading the first and second sets of pilot symbols in the time domain to recover a pilot observation in the time domain and a dense pilot spread in the frequency domain.
 23. The method of claim 22, further comprising: estimating a time-frequency domain joint channel estimation.
 24. The method of claim 21, further comprising: updating a frequency tracking loop with the at least two pilot observations in the time domain.
 25. The method of claim 21, wherein the first set of pilot symbols comprises a pair of pilot symbols received within a first time interval from each other in the time domain, the method further comprising: receiving a third set of pilot symbols at the wireless communications device within a second time interval of the first set of pilot symbols in the time domain; and receiving a fourth set of pilot symbols at the wireless communications device within the second time interval, the second time interval being greater than the first time interval and the third and fourth sets of pilot symbols comprising a second modified common reference signal.
 26. The method of claim 25, further comprising: computing a fine-resolution frequency error of a channel that conveyed the first and third sets of pilot symbols based on an observation of channel variation during the second time interval.
 27. The method of 26, further comprising: de-aliasing the fine-resolution frequency error based on an observation of channel variation during the first time interval.
 28. The method of claim 21, wherein the first and second numbers of resource elements are placed at different subcarriers in the frequency domain, the receiving the first and second sets of pilot symbols further comprising: receiving a first subset of the first set of pilot symbols placed with a first plurality of subcarriers having a first frequency spacing from each other and located within a selected frequency band; and receiving a second subset of the first set of pilot symbols placed with a second plurality of subcarriers having a second frequency spacing from each other and surrounding and including the selected frequency band, wherein the second frequency spacing is greater than the first frequency spacing.
 29. The method of claim 28, further comprising: determining a wideband channel estimate based on de-spreading the received second subset of the first set of pilot symbols; and de-aliasing the wideband channel estimate based on de-spreading the received first subset of the first set of pilot symbols. 